The present invention relates to a television camera which is used as an input device for obtaining picture images and sends out less than 30 frames per second, that is, performs a slow scanning operation, and a television camera which is small in the imaging area scanned with an electron beam.
A large number of television cameras now on the market adopt a scanning system, in which the number of frames per second is 30, the number of scanning lines is 525, and interlaced scanning is carried out. (The above scanning system will hereinafter be referred to as "television standard system"). A frequency bandwidth B required for the above case is given by the following equation: ##EQU1## where m indicates the horizontal resolution, n the number of scanning lines, f.sub.v a vertical scanning frequency, a an aspect ratio, and .alpha. the rate of blanking period in horizontal scanning.
Further, the signal-to-noise ratio S/N of a video signal obtained by a television camera is given by the following equations: EQU S/N=i.sub.s /i.sub.n ( 2) ##EQU2## where i.sub.s indicates a signal current, e the electronic charge, k the Boltzmann's constant, T an absolute temperature, R.sub.L the load resistance of an image pickup tube, R.sub.e the equivalent noise resistance of an FET in the first stage used of a pre-amplifier, and C the stray capacitance connected in parallel with the load resistance R.sub.L and given by the sum of the stray capacitance C.sub.i of the FET and the stray capacitance C.sub.o of the image pickup tube.
In a case where a television camera is used as the input device of an image processor or radiation image detector, it is required to increase the resolution of an image obtained by the television camera and to enhance the signal-to-noise ratio of a video signal sent out from the television camera. In order to increase the resolution of the image, the number of scanning lines in the television camera is increased from 525 to 1,050 or more, and the number of picture elements contained in one picture image is increased from 500.times.500 to 1,000.times.1,000. In this case, when the vertical scanning frequency f.sub.v is made equal to 60 Hz as in the television standard system, the frequency bandwidth B becomes four times larger than that required for the television standard system, and thus the signal-to-noise ratio is reduced.
In order to enhance the signal-to-noise ratio, various countermeasures have been worked out. That is, .circle.1 an increase in signal current (for example, the maximum value i.sub.smax of signal current is increased from 300 nA to a current range from 400 to 500 nA) .circle.2 the use of a pin-lead type image pickup tube having a stray capacitance as small as 2 to 3 pF, .circle.3 the use of an FET having small stray capacitance, and .circle.4 the optimization of a pre-amplifier. However, these countermeasures are insufficient.
Hence, it is necessary to decrease the vertical scanning frequency f.sub.v, that is, to reduce the number of frames per second, thereby preventing the increase in frequency bandwidth B and ensuring a desired signal-to-noise ratio. For example, when the number of frames per second is made equal to 7.5 and the number of picture elements contained in one picture image is made equal to 1,000.times.1,000, the frequency bandwidth B required for this case is about 5 MHz. That is, this frequency bandwidth is substantially equal to the frequency bandwidth required for the television standard system using 525 scanning lines, in spite of the fact that the number of scanning lines is increased to more than one thousand in the above case.
In the above scanning system having high spacial resolution and low time resolution, however, the scanning speed of an electron beam at the surface of a target plate of an image pickup tube is reduced. Owing to such a reduction in the scanning speed, the landing characteristics of the scanning electron beam for the target plate are degraded, and a so-called "water fall" phenomenon is readily generated which is not observed in the scanning operation of the television standard system. The "water fall" phenomenon is a phenomenon that a wave pateern is generated mainly in a peripheral portion of an image and degrades the picture quantity of the image.